Discovery is the part of a lawsuit where you get information together to be able to prove your case at trial. The landlord and his staff will lie, almost without exception, because they want to win. How will disprove what they say? How will you show the judge that your case is solid? Where is the real battle going to be, in the facts or law? You want to know before you go to trial what the landlord is going to claim, and how you are going to answer it.
You are probably aware of depositions, but they are rare in eviction cases because of the time and expense involved. Usually, there are Form Interrogatories and Special Interrogatories [questions],Requests for Admission, and Requests for Production of Documents. There is a 5-day deadline for responding to these, and a Motion to Compel the answers if they fail to respond, incompletely respond, or respond evasively. It is unusual for the landlord to send any discovery to you, but it happens. From your side, it is beneficial to send discovery to the landlord because (1) you generally have the harder and omre complex case to prove where the landlord has a lot of the related information, and (2) it can greatly increase the cost to the landlord of evicting you, planting the seed that this eviction case may cost him far more than it's worth to just work it out with you in a settlement.